Casey, a Pennsylvania Democrat, said Monday he made the shift after reviewing the legal, public policy and civil rights questions involved.

In November, as he successfully sought a second term, Casey said he favored same-sex civil unions but did not take a position on same-sex marriage. He previously has voted to let gays and lesbians serve openly in the military and to repeal the "don't ask, don't tell" policy.

Casey, a Roman Catholic from Scranton, Pa., opposes abortion rights, except in cases of rape, incest or when the life of the mother is in danger.

Several other Democratic senators also recently announced they favor gay marriage.

Same-sex marriage by the numbers

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Same-sex marriage, gay marriage

Today marks the one-year anniversary of the Supreme Court striking down the Defense of Marriage Act, affording equal federal rights and protections to married same-sex couples. Take a look at the numbers behind gay marriage, according to CNN and other sources.